This type of waste heat utilization device for an internal combustion engine includes, for example, a Rankine cycle that is applied to a vehicle equipped with an engine and recovers energy from waste heat generated in the engine, and an ECU (electrical control unit) that controls the Rankine cycle. The Rankine cycle makes a refrigeration circuit where refrigerant runs through a pump, an evaporator, an expander and a condenser in this order.
There is a well-known technology that controls the rotation frequency of the pump to adjust a refrigerant flow rate by means of ECU, adjusts the rotation speed of the expander by controlling the load torque of the expander, and thus enhances recovered energy in the Rankine cycle (see Unexamined Japanese Patent Publication Nos. 2005-30727 and 2006-177266).
One possible way to recover engine waste heat is to provide the Rankine cycle with not only an evaporator but also an exhaust gas heat exchanger for absorbing heat from engine exhaust gas.
However, the above-mentioned conventional technologies do not develop any particular method of controlling the rotation frequency of the pump, which should be employed in the Rankine cycle including the exhaust gas heat exchanger. The enhancement of recovered energy in such a case thus remains an issue.